Commonly in a fibrous-web machine, such as a paper or board manufacturing machine, tail-threading is performed at the normal run speed of the machine. Tail-threading is started from a point in which typically a full-width web is dropped down from a rotating machine element, such as a roll or a dryer, to broke handling, i.e. typically to a pulper or a broke conveyor, by cutting to the web a narrow tail suitable for tail-threading by special cutting devices, which tail also falls before the start of tail-threading to the broke handling similar to the other web. Disadvantageous rotating of the web and the tail around said roll is typically prevented by a doctor knife i.e. trailing knife on the roll surface. The tail-cutting devices affect the run of the tail by cutting it and by directing the new cut tip of the tail to a new path to be conveyed further by other tail-threading devices to a path of the web travelling through the subsequent section of the paper and board manufacturing machine. In this description, the roll side of the tail is referred to as the back side of the tail and the other side its front side. At the front side of the tail, a subsequent section of the paper and board manufacturing machine is located and the tail-threading is performed through it. When the narrow tail has been conveyed through said machine section, it is widened into a full-width web by moving the cutting device by which the tail was formed in the cross machine direction.
There are many and different known tail-cutting devices because the run speed of the fibrous-web machine and the properties of the fiber to be cut, such as paper or board, set very different requirements for the cutting device. The difference between paper and board is not strict but commonly boards are thick and thus flexurally bending stiff compared with paper. Often, the speed of paper machines is also higher than that of board machines.
One type of a cutting device in a paper machine is an air cutting device, by means of which, the tail is released from the surface of the rotating roll before said point in the knife of the doctor knife by blowing air by the so-called releasing blow between the tail and the roll. In the release point of the tail, there is simultaneously arranged a narrow gap between the air cutting device and the roll, whereby the loop of the tail releasing from the roll surface hits the corner of the air cutting device and the upper surface of the device. Close to the corner and on the upper surface of the device, there is arranged an intensive blow blowing away from the roll and the tail enters within the range of a vacuum created by this blow. This blow, which often referred to as the cut-off blow, is typically provided by means of compressed air discharging via a row of holes from a nozzle chamber. The discharging air causes an intensive vacuum at the point of the nozzle holes and the tail is sucked fast in the strongly turbulent nozzle flow which breaks the tail i.e. simultaneously cuts said loop of the tail and the connection between the tail and the tail section going to broke handling. The cut new tip of the tail starts to run in the direction of the cut-off blow and towards the subsequent tail-threading devices. In practice, the operation of the device has been boosted by arranging a similar intensive air blow close to the tail such that the blow aims at conveying the moving tail into the direction preceding cutting towards broke handling. This blow, which is often referred to as a holding-down blow, is positioned in the travel direction of the tail after the said forming loop of the tail and, when the loop comes within the range of the vacuum of cut-off blow, the holding-down blow forms in the tail a force affecting in the opposite direction in relation to the cut-off blow, which in the best case momentarily stops the tail having previously travelled at the run speed of the paper machine. The tail having stopped at the point of the nozzle of the cut-off blow is cut considerably more quickly than the moving tail. The functionality of this type of a cutting device becomes weakened as the rigidity stiffness of paper increases and generally this device is not suitable for cutting board. A more rigid tail will not be released into a loop from the surface of the rotating roll by means of the release blow and the tail will not curve sufficiently to enter within the range of the vacuum of the cut-off blow nozzle.
In board machines, the tail is typically cut between various cutting knives or by guiding the tail against rotating knives. In cutting devices, there is typically a turning cut-off plate, at the tip of which there is the other half of the cut-off knife pair, on the same side of the tail as said rotating roll of which the tail will fall down towards broke handling i.e. on the backside. Before the cutting event, the cut-off plate is directed substantially towards broke handling thus disturbing the run of the tail as little as possible. As the cut-off plate typically turns forward and upward towards the subsequent tail-threading devices at the front of the tail, the cut-off knife of the cut-off plate tip passes by a stationary counter knife, whereby the tail is cut between the knives and the connection between the tail and the tail section going to broke handling is cut.
Another typical structure used is a turning cut-off plate without the knife at its tip when there are quickly rotating knives in place of the counter knife. The cut new tail tip starts travelling into the direction according to the position of the cut-off plate after the cutting moment towards the subsequent tail-threading devices. Typically, there are arranged air blows before the cut-off plate above it, to the cut-off plate itself and after the stationary counter knife, from the effect of which, air blows the travel of the cut tail into the desired direction is more reliable. Due to the location of the device on the same side as the moving roll i.e. at the backside of the tail, the device must be typically located disadvantageously below the doctor knife of the roll. In some cases, lack of available space still prevents the location of the cutting device below the doctor knife, whereby alternative ways must be introduced. However, board cutting has been proved most reliable by devices of the type described above and devices located above the doctor knife are avoided if possible. Such an alternative solution is a combination of a miniature doctor knife, a holding-down blow and a stationary cut-off knife being at its tip pushing under the tail along the roll surface. The cutting is performed in the way of scissors by means of a cut-off knife turning from the front of the tail. It is also typical of known cutting devices that there is precisely designed timing of different blows and motions in relation to each other, which enables their automatic operation.
As a summary, it can be stated that known cutting devices particularly designed for board require a lot of space in the tail-threading section of a fibrous-web machine and are thus awkward to locate or, on the other hand, are relatively complex to implement.
Therefore, there is requirement for improved manipulation devices of fibrous webs and particularly for cutting devices of board webs in the tail-threading section of a board machine.